When you zoom into 1:1 in Library, and assuming that a 1:1 preview doesn't already exist, then obviously a new 1:1 preview needs to be generated. The difference in Library is that the new 1:1 preview is actually a pyramid of differently sized jpegs, which are used in different circumstances (e.g. normal fit/fill loupe mode, grid mode, filmstrip, navigator)....whereas the preview that's built when you load an image into Develop is one file only. So it's logical that zooming into 1:1 in Library could take a little longer than loading an image in Develop, though I agree the difference seems larger that expected. To be honest, I've sometimes noticed that longer-than-expected 1:1 zooming in Library as well, though generally I build 1:1 previews during import, which makes sure that moving through images in Library, even when zoomed to 1:1, is instant.
Bottom line: I don't have an answer as to why the loading gap is so large, but my advice would be to generate 1:1 previews during import, but only if you are regularly needing to view them at 1:1 in the Library module.
Especially with recent updates (I'm on the latest currently) I've always noticed the "loading" box pops up far too frequently and for too long. It seems as if LR is generating a screen image every single time!! Maybe this is by design - I wouldn't put anything past Adobe! But when I use, say Picasa, to view a RAW image and it handles them so fast and smoothly, It isn't just previews (ie doing what it is supposed to do and display a full screen image) but also especially with this latest release, the reaction of sliders is often quite delayed. It really does feel clunky and old. Is LR past ists prime? What will take its place?